CD63 is a Type III membrane protein of the tetraspanin family whose 20 current members are characterized by the presence of four transmembrane segments. Several groups independently identified CD63, using antibodies raised to whole cell preparations of activated platelets, granulocytes, and melanoma cells. Cloning of the respective cDNAs of their cognate glycoprotein antigens led to the recognition that the different antigens were one and the same molecule. The Sixth International Workshop on Leukocyte Typing (1996) subsequently categorized these antibodies as CD63 antibodies. Prior to the 1996 Workshop, CD63 was known by multiple names (melanoma 1 antigen, ocular melanoma-associated antigen, melanoma associated antigen ME491, lysosome-associated membrane glycoprotein 3, granulophysin, melanoma-associated antigen MLA1), which were sometimes related to the antibodies that led to its partial characterization and identification. Thus, CD63 was also designated as antigen ME491 (MAb ME491), neuroglandular antigen (MAbs LS59, LS62, LS76, LS113, LS140 and LS152), Pltgp40 (MAbs H5C6, H4F8 and H5D2), human bone marrow stromal cell antigen (MAb 12F12), osteoprogenitor-specific marker (MAb HOP-26), and integrin-associated protein (MAb 6H1). Other antibodies that were found to cross react with human CD63 were 8-1H, 8-2A (cross-reactivity with ME491), NKI/C-3 and NKI/black-13 (Vannegoor and Rumke, 1986; Demetrick et al., 1992; Wang et al., 1992).
CD63 was initially cloned from a melanoma cDNA library using MAb ME491, one of a number of antibodies raised against a preparation of human melanoma cells. It was shown that the reactivity of MAb ME491 appeared to be inversely correlated with melanoma progression in a study of human melanoma biopsies. The reactivity of the ME491 antibody was low in normal melanocytes, higher in the early stages of melanoma progression (dysplastic nevi and radial growth phase (RGP) tumors) and decreased or even absent in more advanced melanoma tumors such as those in the vertical growth phase (VGP) and in metastatic tumors.
CD63 was also found and partially characterized in human platelets using MAb 2.28 (raised against activated platelets) that detected an activation-dependent platelet membrane 53 kDa glycoprotein. This molecule was also associated with the membrane of internal granules in unstimulated platelets. In the same study MAb 2.28 also labelled internal granules in megakaryocytes and endothelial cells, where it co-localized with antibodies to the enzyme cathepsin D, a known marker of lysosomal compartments. Follow up studies with antibody clustering and expression cloning, led to the identification of the antigen recognized by this antibody as CD63, and further confirmed its presence in lysosomal compartments, where it co-localized with the compartment-specific markers LAMP-1 and LAMP-2. Cloning of this molecule identified it as CD63 and allowed its inclusion in the tetraspanin family.
Expression of CD63 was detected in many different tissues and cell types. At the cellular level it was found to be associated with the plasma membrane and also with intracellular late endosomal vesicular structures. Cell activation led, in certain cases, to increased surface expression by mobilization of intracellular stores of CD63. CD63 was also found to co-localize, and physically associate, with MHC class II in B-lymphocytes, particularly in endosomes, in exosomes involved in exporting MHC class II complexes to the surface, and in secreted vesicles. CD63 was found to interact with other members of the tetraspanin family, such as CD9, CD81, CD 11 (integrin chain αM,L,X), CD 18 (integrin chain β2), CD49c (VLA-3 or integrin chain α3), CD49d (integrin chain α4), CD49f (VLA-6 or integrin chain α6) and CD29 (integrin chain β1), in a variety of cell types including B- and T-lymphocytes, neutrophils, breast cancer and melanoma cells.
The role of CD63 in cancer has been unclear. Although CD63 was initially discovered by several independent groups to be involved in diverse events such as platelet and granulocyte activation, MHC class II-dependent antigen presentation, integrin-dependent cell adhesion and motility, and tumor progression in certain types of cancers, its function has yet to be fully elucidated. Even though current evidence supports its role in a variety of cellular physiological events, it is not clear if these functions are independent of each other or if there is an underlying common cellular mechanism in which CD63 is involved.
Several groups have investigated the association between CD63 and the progression of certain types of tumors, particularly melanomas. A number of other anti-CD63 monoclonal antibodies, in addition to Mab ME491, were developed for immunohistochemical (IHC) staining of cancer samples obtained from patients with tumors at various stages of progression. It was observed that decreased staining, interpreted by the authors as most likely reflecting decreased expression of CD63, correlated with advanced progression and with metastatic characteristics of the tumors. A more recent study, also described a significant correlation between the apparent decreased expression levels (after quantitation of mRNA) of several members of the tetraspanin protein family, including CD63, and the in vitro invasiveness of several mammary carcinoma-derived cell lines. Another study identified CD63, by differential display, in cultured breast cancer cells subjected to estrogen deprivation. This indicated that CD63 expression can be steroid-hormone regulated and that altered CD63 abundance and/or function might also be associated with breast tumor progression.
By contrast, work with anti-CD63 monoclonal antibody MAb FC-5.01 revealed that its reactive epitope was variably expressed in different normal tissues. Although this antibody was found to recognize CD63, it did not distinguish between early and more advanced stage melanomas, including metastatic melanomas (unlike MAb ME491), which suggested that the CD63 antigen was present in these more advanced tumors, but that some of its epitopes may have been masked in the cells from tumors at different stages. This might have been due to altered post-translational modifications of the core CD63 polypeptide, or to the interaction of CD63 with other molecules, which might have affected the availability of specific epitopes for antibody recognition and binding. These results supported the observation, described by Si and Hersey (1993), that staining with the anti-CD63 MAb NKI-C3, did not distinguish between tissue sections from melanomas at different stages of progression, such as primary, radial growth phase, vertical growth phase, and metastatic melanomas. Although in other studies (Adachi et al., 1998; Huang et al., 1998) analysis of mRNA from breast, and from non-small-cell lung cancers, by quantitative PCR, revealed that for two tetraspanin family members (CD9 and CD82) there was a significant correlation between their expression levels and tumor progression and patient prognosis, no such correlation was found for CD63, in that its expression was similar in all the samples. As a result of these, apparently conflicting, results, there is lack of strong and consistent data that would definitively demonstrate the association of CD63 with cancer.
To date very few in vivo studies have attempted to establish a link between CD63 and an eventual tumor suppressor function of this molecule. In one of these studies, human CD63-overexpressing H-ras-transformed NIH-3T3 cells, injected both subcutaneously and intraperitoneally into athymic mice, revealed a decreased malignant/tumorigenic phenotype, as indicated by decreased tumor size and metastatic potential as well as by increased survival time, when compared to the behavior of the parental non-CD63-overexpressing cells. This suggested that the presence of human CD63 in the transformed cells might suppress their malignant behavior. More recently, work with a transgenic mouse model expressing human CD63, and developed to induce tolerance to CD63, indicated that tumor growth of an injected human CD63-MHC class I (H-2Kb) co-transfected murine melanoma cell line could be inhibited, and survival increased, upon immunization with human CD63 fused to vaccinia virus. It was suggested by the authors that the therapeutic effect was T-lymphocyte dependent, and that endogenous anti-CD63 antibodies did not appear to be involved in this protective effect, since tumor growth inhibition only occurred when animals were injected with the CD63-MHC class I co-transfected cells and not with the CD63-only transfected cell line. This interpretation was supported by the fact that in wild type animals, pre-immunized with purified human CD63 and shown to have developed anti-human CD63 antibodies, there was no protective effect against tumor cell growth. Work described by Radford et al. (1995) using the KM3 cell line, initially thought to be of human origin but later characterized as being of rat lineage, transfected with human CD63, suggested that expression of this protein decreased the growth and metastastic potential of these cells, relative to that observed using the parental non-transfected KM3 cells, when injected intradermally into athymic mice, although there was no significant difference between the in vitro growth rates of the various transfected and non-transfected cell lines. These observations distinguished the potential effect of CD63 from that of other tumor suppressor genes known to affect both the in vivo and the in vitro growth rates of tumor cells. Furthermore, addition of the anti-CD63 monoclonal antibody ME491, which was found to have a functional effect on the same cells by decreasing their random motility in an in vitro assay (Radford et al., 1997), did not impact their in vitro growth rates.
This study also described the observation that CD63 may promote migration in response to extracellular matrix (ECM)-derived chemoattractants, such as laminin, fibronectin, collagen and vitronectin, and that this effect may be mediated by the functional involvement of β1-type integrins, although antibodies to the integrins were unable to block these effects. However, there appeared to be an antagonistic effect between the role of vitronectin-mediated signaling (a known ligand for the integrin αvβ5) and that of the signaling mediated by other ECM components such as fibronectin, laminin and collagen on CD63 transfected cells. This suggested that under specific conditions, in the presence of ECM components, expression of CD63 may lead to decreased migration, and that this may be dependent on a fine balance between adhesion and motility. In another study, an anti-CD63 monoclonal antibody (MAb 710F) enhanced the adhesion and spreading of PMA-treated HL-60 cells, while another anti-CD63 monoclonal antibody (MAb 2.28), promoted a similar effect, but only on a much smaller fraction of the cell population, and only when added in much larger amounts. These results showed that although many antibodies to CD63 have been developed, their functional effects can be quite different.
Tetraspanins may also be involved in cell proliferation. Oren et al. (1990) described anti-proliferative effects of the murine MAb 5A6, that recognizes CD81 (TAPA-1), on lymphoma cell lines. In another study, ligation of CD37 in human T-lymphocytes with antibodies blocked CD37-induced proliferation. More recently, a study with an animal model deficient in the expression of CD37 (CD37 knockout) revealed that T lymphocytes from this animal were hyperproliferative compared to those from wild type animals in response to concanavalin A activation and CD3/T cell receptor engagement. It was therefore proposed that a functional role in cell growth and proliferation might be a common feature of the tetraspanin family. Recent studies with hepatoblastoma and hepatocellular carcinoma cells revealed that engagement of these cells with anti-CD81 monoclonal antibodies led to activation of the Erk/MAP kinase pathway. This signaling pathway has been shown to be involved with cell growth and proliferation events. In parallel work, transfected cell lines overexpressing human CD81 displayed increased proliferation relative to the mock-transfected control cells. Therefore, available evidence has pointed to a role of the tetraspanins in general, and of CD63 in particular, in events associated with cell growth proliferation and with cell adhesion/motility. These two types of cellular events are currently the target of intense research as both play a central role in tumor progression and metastasis.
Until now, no anti-CD63 antibodies, or other reagents that specifically targeted CD63-expressing cells, were reported and shown to have a simultaneous impact on the in vitro and on the in vivo growth characteristics of tumor cells, and also on the survival time of animal models of tumor cell growth.
Amino acid sequence determination and analysis did not reveal homology between tetraspanins and other protein families, or with any previously characterized functional modules, nor has it suggested any previously known enzymatic activity. As a result it has been very difficult to investigate the role of this family of proteins in the modulation of signal transduction pathways. However, the evidence generated using tetraspanin-specific reagents that led to changes in cellular physiology, and which were intimately dependent on the modulation of signal transduction pathways, suggests that tetraspanins have signal transduction properties. CD63 was shown to associate, both physically and functionally, with a number of molecules that are themselves either enzymes involved in the generation of secondary messenger signals, or are associated physically and/or functionally with such enzymes.
Experiments designed to dissect the mechanism controlling the interaction of human neutrophils with endothelial cells, which is one of the initial steps of the inflammatory response, revealed that pre-treatment of neutrophils with several anti-CD63 monoclonal antibodies (AHN-16, AHN-16.1, AHN-16.2, AHN-16.3 and AHN-16-5) promoted their adhesion to cultured endothelial cell layers. Furthermore this effect was strongly dependent on the presence of calcium ion (Ca2+), a well-known modulator of many intracellular signaling pathways and which was restricted to a specific period of time during which the cells were exposed to the stimulating antibodies. After longer exposure to the antibody, adhesion of the neutrophils to the endothelial cells became insensitive to the later addition of Ca2+, therefore implicating a dynamic and temporally regulated (transitory) event. In addition, CD63 was found to physically interact with the CD11/CD18 protein complex, and reagents that specifically targeted this complex mediated a modulatory signal. In this study CD63 was also found to be physically associated with, or to be part of, a complex that included the enzyme tyrosine kinases Lck and Hck. These enzymes are members of a class of proteins that play a central role in mediating intracellular regulatory signals upon activation of specific surface receptors and are part of cascades of signaling pathways that result in cell-specific physiological changes. Another study suggested that co-ligation of tetraspanins (including CD63) with monoclonal antibodies could enhance the phosphorylation or activity of the enzyme focal adhesion kinase (FAK) that was induced by adhesion of MDA-MB-231 breast cancer cells to collagen substrate. This pointed to a direct involvement of CD63 (and of other tetraspanin family members) in the modulation of integrin-mediated tyrosine kinase signaling pathways. Other signaling pathways that may functionally intersect with the presence and ligation of surface CD63 by the anti-CD63 monoclonal antibody MAb 710F appear to be those dependent on modulation of phosphorylation by the enzyme protein kinase C (PKC), another well known modulator of intracellular signaling pathways. In this context, enhancement of adhesion and of morphological changes in the myeloid cell line HL-60 by MAb 710F was dependent on pre-treatment of the cells with phorbol myristate acetate (PMA) although the temporal involvement of PKC was not conclusively demonstrated. However, later work by an independent group demonstrated that PMA-induced HL-60 differentiation was PKC-activity dependent since the molecule Ro31-8220, a specific inhibitor of this enzyme, blocked the effect of PMA.
Further evidence supporting the association of CD63, and other tetraspanin family members, with signal transduction pathways, arose from work that described a physical association, either direct or as part of a supramolecular complex, between CD63 (and also CD53) molecules with tyrosine phosphatase activity. In this study, immunoprecipitate complexes isolated with anti-CD63 antibodies were shown to be associated with tyrosine phosphatase activity, although unlike for CD53, which was shown to associate with the tyrosine phosphatase CD45, it was not possible to identify the CD63-associated phosphatase. More recently several members of the tetraspanin family were also found to be associated with a type II phosphatidylinositol 4-kinase (type II PI 4-K) (Berditchevski et al., 1997). This interaction appeared to be very specific since it was only identified for CD9, CD63, CD81, CD151 and A15/TALLA, and it was not observed to occur with CD37, CD52, CD82, or NAG-2. In addition, the association between tetraspanin family members and PI-4K was mutually exclusive since each PI-4 kinase-containing complex was limited to a single tetraspanin family member. CD63-PI-4 kinase complexes, in particular, were found, almost entirely, in intracellular compartments in lipid raft-like domains, unlike those formed with the other tetraspanin members. This observation suggested that this CD63 fraction, found to interact with the PI-4 kinase, might have been involved in specific intracellular events (Claas, C, et al., 2001) related to, or dependent from, phosphoinositide biosynthesis pathways, which are well known for their involvement in the regulation of membrane trafficking (endocytosis and exocytosis) and of cytoskeleton reorganization, in addition to their function as secondary messenger molecules (Martin, T., 1998).
The direct and important involvement of all the enzymes, that CD63 was found until now to be directly associated with, in the regulation of signaling pathways provided further evidence in support of the association of CD63 with the modulation of signal transduction pathways, either as a regulator or as an effector molecule downstream from the activity of these enzymes.
Elucidation of the mechanisms that lead to tumor progression is a very difficult and complex endeavor frequently marked by apparently contradictory observations and, as a result, it rare that those observations successfully translate into effective therapies. In view of what is currently known about the association of CD63 with tumor progression and metastasis and with signal transduction mechanisms, it is possible that its function may be altered, in tumor cells.
Development of antigen-specific reagents with cytotoxic effects on tumor cells, that bind cells expressing the recognized antigen(s) and which by themselves, or associated with other molecules, have cellular and in vivo physiological activity such that these reagents inhibit tumor cell growth, progression and metastasis, without significant deleterious effects on normal cell populations, would be extremely beneficial as a potential therapeutic and or diagnostic tool.
Prior Patents:
U.S. Pat. No. 5,296,348 teaches methods for selecting monoclonal antibodies specific for cancer cell surface antigens that are internalizing, and for identifying monoclonal antibodies having anti-transcriptional and/or anti-replicational effects on cell metabolism. By way of example the ME491 antibody was shown to internalize in W9, WM35, WM983 melanoma cells, and SW948 colorectal carcinoma cells. In addition ME491 antibody was shown to decrease transcription and cell proliferation in SW948 cells. The patent application US20030211498A1 (and its related applications: WO0175177A3, WO075177A2, AU0153140A5) allege a method of inhibiting the growth or metastasis of an ovarian tumor with an antibody that binds an ovarian tumor marker polypeptide encoded by an ovarian tumor marker gene selected from among a group that includes CD63 antigen. Serial analysis of gene expression using ovarian cancer was carried out to identify ovarian tumor marker genes which lead to the identification of CD63 as a candidate. The patent application WO0205555 A1 (and its related application CN1364803A) alleges a new polypeptide-human CD63 antigen 56.87. The patent application CN1326962A alleges a new polypeptide-human CD63 antigen 14.63. The patent application CN1326951A alleges a new polypeptide-human CD63 antigen 15.07. The patent application CN1351054A alleges a new polypeptide-human CD63 antigen 11.11. These patents and patent applications identify CD63 antigens and antibodies but fail to disclose the isolated monoclonal antibody of the instant invention, or the utility of the isolated monoclonal antibody of the instant invention.